Question 1040087
It is [ln(.298)/-0.693]*5730=1.747*5730=10010.2 years
-0.693 is ln (1/2), the half life, or 50%. If 75% were lost, that would be 2 half-lives or 11460 years, so this is in the range. 
Determine the number of half life (1/2)^n=0.298
ln of both sides is n ln(1/2)=ln 0.298.  Divide both sides by ln (1/2), which is -0.693